Total Party Kill. When the opponents of the player characters end up killing all the player characters. Often unexpected, and sometimes blamed on poor understanding by the GM of difficulty level of the opponents.
A new game system that closely emulates an older edition of a game system that is out of print in order to compatible with anything produced for the older system. Often, but not always, derived from Open Gaming materials under the Open Gaming License (OGL) or similar public copyright licenses.
Labyrinth Lord (LL1, LL2, LL3) and Old-School Essentials (OSE1, OSE2) are retro-clones of B/X D&D (B1, B2, X1, X2). Swords & Wizardry Complete (S&W1, S&W2) is a retro-clone of Original D&D plus some additional material from The Strategic Review and Dragon magazines. OSRIC is a retro-clone of Advanced Dungeons & Dragons, first edition.
A new game system that is inspired by an older edition of a game system, but which uses somewhat different mechanics.
Dan Proctor has a useful discussion of neo-clones and retro-clones and what he calls the neo-retro.
A broad category of game systems that claims a “retro” or “old-school” feel. It stretches to include games such as the Dungeon Crawl Classics RPG or Mazes & Minotaurs which include mechanics that are considerably different from retro-clones and the games that inspired them.
“A megadungeon is both a campaign structure and a sort of exceptionally large and complex dungeon that a party may explore as part of a campaign using that structure.” (M1, M2, M3, M4) DMDavid has some interesting articles on them: DMD1, DMD2, and DMD3.
A renewable resource that a player can spend so their character can do something they otherwise couldn't do, like reroll something or convert a damage strike into a flesh wound or buying a clue or increase a die roll before or after the fact. Examples are Force Points in Star Wars: the Roleplaying Game, Hero Points in Mini Six, Fate Points in OpenD6, Bennies in Savage Worlds, Inspiration in D&D 5E.